Treatment of benzene hexachloride



United States Patent 2,717,272 TREATMENT OF BENZENE HEXACHLORIDE PercyW. Trotter, Baton Rouge, La., assignor to Ethyl Corporation, New York,N. Y., a corporation of Delaware No Drawing. Application July 19, 1952,Serial No. 299,926

4 Claims. (Cl. 260-648) This invention relates to the treatment of anisomeric benzene hexachloride mixture to enhance the gamma isomerconcentration therein and more particularly to a process for separatingthe gamma and delta isomers whereby the gamma isomer may be recoveredfrom isomeric mixtures in high yields.

It is known that benzene hexachloride (1,2,3,4,5,6-hexachlorocyclohexane), hereinafter called BHC, occurs in a number ofisomeric forms. For example, in .the addition chlorination of benzene,at least five such iso mers are formed having approximately thefollowing weight distribution: alpha, 65 percent; beta, 8 percent;gamma, 13 percent; delta, percent; and epsilon, less than one percent.It is also known that the gamma isomer is by far the most insecticidallyactive BHC isomer. Accordingly, it is desirable to concentrate isomermixtures with respect to the gamma isomer to facilitate preparation ofimproved insecticidal formulations by the elimination of inert andodoriferous constituents thereof.

Prior methods have been disclosed for concentrating the gamma isomer inBHC isomeric mixtures by extraction and fractional crystallization from.a solvent having different solubilities for the various isomers. Theextraction is carried out using only sufiicient solvent to dissolve thegamma isomer while leaving considerable quantities of the relativelyinsoluble alpha isomer undissolved. Separation of the undissolved alphaisomer can be accomplished by centrifugation, filtration or the like.The gamma isomer can be recovered, in part, from the more soluble deltaand epsilon isomers by inducing supersaturation of the gammaisomer-containing extract and Without inducing substantialsupersaturation of the delta and epsilon isomers and concurrentlyforming .a crystal crop high in gamma isomer. This induction of a stateof super-saturation can be accomplished by concentration of the extract(removal of the solvent), temperature reduction .(cooling the extract),addition of a solvent depressant (addition of water to methanol solvent,for example), or any combination of the above. In any event, however,the yield of relatively pure gamma isomer from BHC isomeric mixtures islimited by the delta isomer concentration in the extract. In otherwords, the degree of super-saturation induced in the extract determinesthe yield of the gamma isomer, but the degree of supersaturation inducedmust be limited so as to prevent crystallization of the delta isomer.Thus the gamma/delta solubility ratio at the crystallizing temperaturein any solvent and the weight distribution of the isomers in theisomeric mixture determines the maximum yield obtainable in any knownextraction and .crystallization process. Using crude BHC, the optimumyield of gamma isomer is about 80 percent, not considering the loss inmechanical separation and the like. If the delta isomer is permitted tocrystallize, it is extremely difficult to further purify the gammaisomer product by recrystallization.

In view of the above, it has been found necessary to discardconsiderable portions .of the gamma isomer in the mother liquor from thecrystallization operation, due

2,717,272 tented Sep 1955 2 to its high concentration of delta isomerand other soluble constituents. This loss accounts for the major gammaloss .in processes of this type. Up to the present, there is no suitablemeans for treating this high delta stream to recover its gamma content.a

It is accordingly an object of thisinvent-ion to provide a process forseparating the gamma and delta fbenzene hexachloride isomers, therebypermitting the recovery of gamma isomer in high yields.

Another object of the invention is to provide a process of the abovetype, in which a relatively pure gamma isomer fraction may be obtained.

Another object of this invention is to provide a process of the abovetype which is adapted to economical large scale commercial operation.

Other objects and advantages of this invention will become apparent asthe description proceeds.

It has now been found that BHC isomer mixtures containing relativelyhigh concentrations of the delta isomer, relative to the gamma isomer.(above one part of delta isomer to two parts of gamma isomer), can beseparated when the BHC is in a state of partial fusion. Using'thistechnique, a major portion of the gamma isomer previously lost can berecovered as a liquid fraction having a relatively low delta isomerconcentration. It has fur ther been found that this separation techniquecan be combined with any known extraction and crystallization processesso as to recover, in contrast with prior processes, essentially all ofthe gamma isomer normally lost in the high-delta-isomer crystallizermother liquor.

The separation of the partially fused -BHC is carried out without asolvent, although the presence ,of a small amount of solvent does notimpair the process. The temperature of separation can be between aboutroom temperature and 1-00 C. and is preferably carried out between about30 and 85 C. Also, the separation can be carried out at a singletemperature within the noted above. However, preferably ,the material.being treated is gradually heated or heated step-wise from a temperature.of about 30 to about When a small amount of solvent is present, thetemperatures are corresp ondingly lowered.

7 Although the theory-of the above proeessis notentirely understood, itis believed that low melting point eutectic mixtures are formed betweenthe various isomers. One of the lowest melting eutectic mixturesincludes the delta and gamma isomer and is formed in a ratio o f .-about:two parts gamma isomer to one part delta isomer. other slightly highermelting eutectic mixtures are also formed between the gamma isomer and.other isQmcr-s. Eutectic mixtures having melting temperatures aboveabout 1:09" C. have been found much less favorable tosgammaisomerrecovery. Thus, if the temperature of separation is' raised materiallyabove C., these other eutectic mixtures are also recovered in thegamma-containing filtrate and, thus, essentially no concentration of thegamma isomer is obtained. In view of the above, .it.,ha s been foundpreferred to begin .the separation .at about room temperature and,graduallyor periodically .raise the temperature to between about -.801.00- .C., while .continuously separating and recovering the liquidphase-I The separation of the fused eutectic mixture can be elfected byany suitable filter or separatory device. However, since eflicientseparation is desired, a centrifuge or the like is most preferred.

The present process for separatingthe deltaand gamma isomer can 'becarried out in combination with any :knQWn extract-ion.andcrystallization process. :Usually, after crystallization .of .ahigh-gamma fractionfrom an extract solution, the mother liquor from ,thecrystallization ,is desolventized and the solute solids recovered. Sucha fraction is a particularly suitable feed for the present process. Thismixture ofisomers of BHC is maintained at or brought within thetemperature range of operation, forming a fused or lquid phase. Thisphase is separated by any efficient liquid-solid separation unitoperation. The liquefied phase so-obtained is substantially enriched inthe valuable gamma isomer.

The filtrate obtained from this separation, if desired, is thenfurthertreated to concentrate the gamma isomer, either by a separateprocess or by recycling the filtrate to the extractor-or to thecrystallizer. if the alpha concentration in the centrifuge filtrate isrelatively high, it is generally preferred to recycle the material tothe extractor so as to eliminate excess quantities of this isomer in thehigh alpha extraction cake. If desired, however, the filtrate havingexcess alpha can be extracted by fresh solvent to eliminate theundesired alpha directly. When the alpha/gamma ratio is suflicientlylow, the filtrate tram the centrifugation or other separation means canbe directly recycled to the crystallizer.

The feed for the above process may vary widely in isomer distributionbut is particularly suited to high delta isomer feed stocks. Thus, thisinvention has a peculiar suitability for use in treating the high deltacrystallizer mother liquor from prior extraction and crystallizationprocesses. To best illustrate the advantages of the present process, theexamples include a description of an overall process in which crude BHCis first extracted with methanol and a high gamma isomer fractioncrystallized therefrom.

The following are typical examples of the present invention. Allquantitative units are given in parts by weight.

EXAMPLE I Crude BHC was extracted at C. in a continuous process withmethanol using a feed ratio of 1.8 parts of methanol to one part of BHC.The extract was concentrated by removal of 60 percent of the methanol.The concentrated extract was then crystallized at 15 C. and uponseparation of the crystals, a portion of the mother liquor was recycledto the extractor. At steady state conditions, three-quarters of themother liquor was recycled and one-quarter of the recycle was treated asfollows: the methanol solvent was completely removed from the solidsfraction of the mother liquor. The solids fraction comprised a BHCisomeric mixture (175 parts) having 36 percent delta isomer and percentgamma isomer. This mixture was gradually heated over a two-hour periodfrom 40 to 80 C. and the liquid fraction continuously recovered in acentrifuge. The filtrate so-obtained contained 72 percent of thesolvent-free BHC and 34 percent gamma isomer, 18.5 percent delta isomer,10.5 percent alpha isomer and the remaining primarily beta isomer andheptachlorocyclohexanes. The cake obtained in this centrifuge contained2.8 percent gamma isomer and approximately 80 percent delta isomer.

EXAMPLE II A BHC isomer mixture (135 parts) containing 36 percent deltaisomer, 25 percent gamma isomer, 10.5 percent alpha isomer, and theremainder primarily beta isomer and heptachlorocyclohexanes, wascentrifuged, using 750 gravities. The temperature during thiscentrifugation was gradually raised from 40 to 85 C. over a two-hourperiod. The filtrate (93.1 parts) had a gamma isomer purity of 32.8percent corresponding to 91 percentyield. The filtrate also contained21.7 percent of the delta isomer.

The important significance of the high gamma isomer recovery of thisexample, of course, is that the feed material in this embodimentrepresents, normally, a major loss stream in a process of extraction ofcrude BHC and crystallization of a high gamma fraction. By providing arecovery of the order of 90 percent of this formery discarded gammafraction, the commercial merits of the process are obviously great.

EXAMPLE III A BHC isomer mixture (105 parts) containing 36 percent deltaisomer, 25 percent gamma isomer and 10.5 percent alpha isomer wascentrifuged using 750 gravities. The centrifugation was .started at 30C. and the temperature was gradually raised to 130 C. over a two-hourperiod. The-following table gives the results obtained for cuts atvariations in temperature.

Table Percent; Percent Percent 0 filtrate gamma of total of totalTemperature C. (Parts) in filfiltrate gamma in delta in trate outfiltrate filtrate out cut out Small 23. 9 29. 0 17. 6 26 4 11. 1 21 327. 5 l9. 1 22. 3 10. 8 12. 4 34 5 23. 8 16.3 7. 8 2. 5 40. O 28. 6 19.0 9. 5 3. 5 36. 0 35. 5 4. 8 3. 3 6. 5 4. 2 82.0 0.6 14. 1

This table shows that up to C. approximately, percent of the gammaisomer is recovered in the filtrate whereas only about 45 percent of thedelta isomer is present therein. However, when the temperature is raisedabove 85 C. and particularly about C., the unde sired delta isomerrecovery increases considerably whereas only small additional recoveriesof gamma isomer are obtained. It will be noted that the filtraterecovered up toat least 85 C. has a gamma/ delta ratio essentially thesame as the gamma/ delta ratio in crude BHC and therefore essentiallyall of this filtrate may be treated, such as by crystallization, in amanner similar to crude BHC.

As is believed apparent from the foregoing, the present inventionprovides an economical and efiicient method for separating the gamma anddelta isomers of benzene hexa'chloride. This process is applicable tothe treatment of essentially all isomer mixtures of benzenehexachloride. It is particularly useful, however, in treating mixtureshaving a high delta/gamma ratio. For this reason, the present processmay be efiiciently and conveniently used to treat the crystallizermother liquor of prior known extraction and crystallization processes torecover the gamma isomer therefrom. As pointed out above, thiscrystallizer stream has been a total loss in prior processes and thepresent invention provides a means to recover at least 90 percentthereof as a valuable gamma isomercontaining material. While not limitedthereto, the sotreated gamma isomer-containing material may be recycledto the'process from which it was obtained. This means that in allprocesses which are limited by the concentration and solubility of thedelta isomer, considerably increased yield of the gamma isomer can beobtained.

I claim:

1. Aprocess for recovering the gamma isomer from an essentiallysolvent-free mixture of benzene hexachloride isomers, including thegamma, alpha and delta isomers, and having at least one part of deltaisomer per two parts of gamma isomer comprising heating said mixcure toa temperature Within the range of from 30 to 100 C., separating theliquid phase formed while the mixture is within said temperature rangefrom the solid phase, said liquid phase containing principally the alphaand gamma isomers and said solid phase containing principally the deltaisomer.

2. In a process for recovering the gamma isomer from a crude benzenehexachloride isomeric mixture including extracting said mixture withsufiicient solvent to dissolve essentially all of the gamma and deltaisomers and leaving a substantial quantity of the alpha isomerundissolved, eltecting crystallization of a part of the gamma isomerfrom the so-formed extract, and separating the gamma s isomer crystalsfrom the remaining mother liquor solution, the improvement comprisingrecovering the dissolved benzene hexachloride containing gamma, alphaand delta isomers from said mother liquor solution, heating the latteressentially solvent-free benzene hexachloride to a temperature withinthe range of from 30 to 100 C., and separating the liquid phase formedwithin said temperature range from the solid phase, said liquid phasecontaining principally the gamma and alpha isomers and said solid phasecontaining principally the delta isomer.

3. A process in accordance with claim 1 wherein a liquid phase of saidmixture formed at a temperature 6 between 30 C. and 85 C. is separatedfrom the solid phase.

4. A process in accordance with claim 2 wherein said recovered liquidfraction is recycled to said crystallization 5 step.

References Cited in the file of this patent UNITED STATES PATENTSGreenburg Apr. 16, 1946 Cooke et a1 Apr. 6, 1948

1. A PROCESS FOR RECOVERING THE GAMMA ISOMER FROM AN ESSENTIALLYSOLVENT-FREE MIXTURE OF BENZENE HEXACHLORIDE ISOMERS, INCLUDING THEGAMMA, ALPHA AND DELTA ISOMERS, AND HAVING AT LEAST ONE PART OF DELTAISOMER PER TWO PARTS OF GAMMA ISOMER COMPRISING HEATING SAID MIXTURE TOA TEMPERATURE WITHIN THE RANGE OF FROM 30* TO 100* C., SEPARATING THELIQUID PHASE FORMED WHILE THE MIXTURE IS WITHIN SAID TEMPERATURE RANGEFROM THE SOLID PHASE, SAID LIQUID PHASE CONTAINING PRINCIPALLY THE ALPHAAND GAMMA ISOMERS AND SAID SOLID PHASE CONTAINING PRINCIPALLY THE DELTAISOMER.